http://www.bbc.co.uk/rd/pubs/reports/1972-25.pdf

Also if you are interested in why the BBC no longer uses their LS series of
speakers, you can find this document on their web site.
The selection of loudspeakers for BBC Radio & Music.

In essence, speaker technology improved and they were able to buy systems
that were better and more accurate. Naturally, they used DBT's to select
new speakers.